NAT before IPSEC - reply packets stuck at enc0

[Muenz, Michael]

Am 25.07.2017 um 10:43 schrieb Muenz, Michael:
> Am 25.07.2017 um 10:22 schrieb Andrey V. Elsukov:
>>
>> ICMP request should be matched by outbound IPsec policy. Looking to your
>> tcpdump, you use tunnel IPsec mode. So, how this should work:
>>
>> * 10.26.2.N sends ICMP request to 10.24.66.25
>>
>> * 10.26.1.1 handles it by tunnel mode IPsec security policy, 
>> something like:
>>     spdadd -4 10.26.2.0/24 10.24.66.0/24 any -P out ipsec \
>>         esp/tunnel/213.244.192.191-81.24.74.3/require;
>> * IPsec code does lookup for IPsec SA and uses something like:
>>     add 213.244.192.191 81.24.74.3 esp 0x2478d746 -m tunnel -E ...;
>
> Thanks for the detailed explaination! I only know the insights with 
> Linux, but what I try to achieve is, not to build a SA fpr 10.26.2.0 
> to 10.24.66.0.
> So IMHO the address rewriting from 10.26.2 to 10.26.1 should be done 
> before getting to the IPSEC process.
> In Linux a packet not matching a SA would simply be dropped by kernel 
> or throw a "NO PROPOSAL CHOSEN" since there's no known SA for 
> 10.26.2.0 to 10.24.66.0.
>
> I'll try to reach out the OPNsense guys if they are willing to patch a 
> new kernel for me.
>
> Thanks!
>
> Michael 

This is the output with the new kernel:

14:02:53.960436 (authentic,confidential): SPI 0xdeda7104: IP (tos 0x0, 
ttl 63, id 6287, offset 0, flags [none], proto ICMP (1), length 28, bad 
cksum b07 (->c07)!)
     10.26.1.1 > 10.24.66.25: ICMP echo request, id 38600, seq 0, length 8
14:02:53.960460 (authentic,confidential): SPI 0xdeda7104: IP (tos 0x0, 
ttl 64, id 32607, offset 0, flags [none], proto IPIP (4), length 48, bad 
cksum 0 (->c99b)!)
     213.244.192.191 > 81.24.74.3: IP (tos 0x0, ttl 63, id 6287, offset 
0, flags [none], proto ICMP (1), length 28)
     10.26.1.1 > 10.24.66.25: ICMP echo request, id 38600, seq 0, length 8
14:02:53.968634 (authentic,confidential): SPI 0xcdea472d: IP (tos 0x0, 
ttl 58, id 18352, offset 0, flags [none], proto IPIP (4), length 48)
     81.24.74.3 > 213.244.192.191: IP (tos 0x0, ttl 63, id 38328, offset 
0, flags [none], proto ICMP (1), length 28)
     10.24.66.25 > 10.26.1.1: ICMP echo reply, id 38600, seq 0, length 8
14:02:53.968653 (authentic,confidential): SPI 0xcdea472d: IP (tos 0x0, 
ttl 63, id 38328, offset 0, flags [none], proto ICMP (1), length 28)
     10.26.1.1 > 10.26.1.1: ICMP echo reply, id 44919, seq 0, length 8

So the most specific nat rule in order to get the packet into enc0 is:

ipfw nat 1 config ip 10.26.1.1 log reverse
ipfw add 179 nat 1 log all from 10.26.2.0/24 to 10.24.66.0/24 in recv vtnet1
ipfw add 179 nat 1 log all from 10.24.66.0/24 to 10.26.1.1 in recv enc0


Thanks!

Michael